1. Field of the Invention
The present invention relates to a magneto-optic recording apparatus of a magnetic modulation scheme and a method of recording information on a magneto-optic recording medium using this apparatus.
2. Related Background Art
FIG. 1 is a circuit diagram showing the arrangement of a general conventional magnetic head apparatus. Referring to FIG. 1, this apparatus comprises a magnetic head 1 and auxiliary coils L.sub.1 and L.sub.2. The magnetic head 1 is actually constituted by magnetic cores (not shown) and coils wound around the cores. Switching elements T.sub.1 and T.sub.2 must switch a current of 100 mA or more within a very short period of time. To satisfy this requirement, the switching elements T.sub.1 and T.sub.2 are optimally constituted by field effect transistors, respectively. In the following description, the field effect transistors T.sub.1 and T.sub.2 will be briefly referred to as transistors, respectively. Gate drive circuits 7 and 8 drive the gates of the transistors T.sub.1 and T.sub.2, respectively.
In the magnetic head drive apparatus described above, a digital information signal S.sub.1 and its inverted information signal S.sub.2 are input to the gate drive circuits 7 and 8, respectively. The gate drive circuits 7 and 8 generate gate drive voltages in correspondence with the information signals S.sub.1 and S.sub.2 of logic "1" to alternately turn on the transistors T.sub.1 and T.sub.2 in accordance with the information signals. The direction of current in the magnetic head 1 is switched in correspondence with logic "1" and logic "0" of the information signals, so that the magnetic head 1 generates magnetic field components having different polarities respectively corresponding to the current directions. More specifically, a magnetic field generated by the magnetic head 1 is modulated in accordance with the information signals, and the modulated magnetic field is applied to a magneto-optic recording medium (not shown). Magnetization patterns corresponding to the polarities of the applied magnetic field components are formed on the recording medium, thereby recording the information signals. The auxiliary coils L.sub.1 and L.sub.2 can always receive current regardless of the ON/OFF states of the transistors T.sub.1 and T.sub.2. The auxiliary coils L.sub.1 and L.sub.2 operate to reverse the current supplied to the magnetic head 1 at high speed.
FIGS. 2A to 2C are views showing a relationship between the modulated magnetic field components of the magnetic head and the magnetization patterns recorded by these modulated magnetic field components. FIGS. 2A to 2C exemplify recording of a single frequency signal with a 50% duty on the magneto-optic recording medium. FIG. 2A shows a magnetic field generated by the magnetic head. The absolute values of the positive and negative magnetic field strengths are .+-.H.sub.0 because the magnitudes of currents in the magnetic head in both directions are equal to each other. FIG. 2B shows a magnetization pattern obtained upon application of the modulated magnetic field to the magneto-optic recording medium. When the positive magnetic field is applied to the magneto-optic recording medium, an information pit having upward magnetization can be recorded. When the negative magnetic field is applied to the magneto-optic recording medium, an information pit having downward magnetization can be recorded. In FIG. 2B, a pit represented by + has upward magnetization, whereas a pit represented by - has downward magnetization. The magnetic head serving as an inductance element requires a certain switching time, e.g., about 25 ns. Assume that magnetic field components required for recording are defined as .+-.H.sub.th. When magnetic field components generated by the magnetic head reach .+-.H.sub.th, the magnetization pattern shown in FIG. 2B is recorded. FIG. 2C shows a reproduction signal obtained by reproducing this magnetization pattern. In this case, the lengths of upward magnetization and downward magnetization and the lengths of reproduction signals T.sub.+1 and T.sub.-1 are equal to each other because the magnetic field components from the magnetic head are directly applied to the recording medium.
In the conventional magneto-optic recording apparatus, however, leakage magnetic field components from magnetic field sources arranged near the magnetic head adversely affect the magnetic field components from the magnetic head, and the magnetic field components from the magnetic head are not always applied directly to the recording medium. The magnetic field sources are, for example, an actuator for driving an objective lens for focusing a light spot on the recording medium and a linear motor for accessing the optical head to a desired position on the recording medium. The leakage magnetic field components cannot be neglected because recent magneto-optic recording media have a higher sensitivity corresponding to a magnetic field sensitivity of 100 Gauss or less due to low power consumption and high-speed operation of the magnetic head. These leakage currents adversely affect the magnetic field components of the magnetic head.
If a leakage magnetic field of a linear motor or the like is defined as +H.sub.e and magnetic field components generated by a magnetic head are defined as .+-.H.sub.0, a magnetic field applied to the recording medium becomes a sum of the leakage magnetic field +H.sub.e and the modulated magnetic field components .+-.H.sub.0, as shown in FIG. 3A. Note that magnetic field components .+-.H.sub.th are magnetic field components required for recording. The positive and negative magnetic field strengths on the recording medium are different from each other. When information is recorded under this condition, the length of upward magnetization of the magnetization pattern recorded on the recording medium is different from that of the downward magnetization thereof, as shown in FIG. 3B. That is, the magnetic field applied to the recording medium is shifted to the positive side due to the influence of the leakage magnetic field components, and the lengths of domains change accordingly. When the recorded information is reproduced, the pulse widths of the reproduction signal correspond to the lengths of the upward magnetization and the downward magnetization. A pulse width T.sub.+2 of "1" is different from a pulse width T.sub.-2 of "0", and a deviation from the duty of the original recording signal has occurred. This phenomenon depends on the magnitudes and directions of the leakage magnetic field components. The magnitude and direction of the duty vary depending on the positions of the leakage magnetic field sources such as the linear motor and the variations in leakage magnetic field components. When duty errors increase, recording error may occur.